The present invention relates to coatings having a continuous compositional gradient and methods for their preparation. The present invention further relates to the formation of stable interfaces between two materials having large differences in their physical properties, specifically, thermal expansion coefficients.
For many applications, i.e., catalysts, wear-resistant and tribological articles, it is necessary to join two materials with very different physical characteristics. This is particularly the case for ceramic-coated metals. The differences in thermal expansion coefficients and ductility makes the materials particularly susceptible to mechanical and thermal shock leading to delamination or spalling of the coating layers.
In an attempt to alleviate this problem, an interlayer with intermediate chemical and physical properties is used. As a further refinement of this process, several layers with varying physical and chemical properties are applied between the substrate and coating.
U.S. Pat. No. 3,620,808 discloses the formation of a thermal emissivity coating on a metallic substrate. To reduce thermal shock and improve handleability, several discrete coatings, each containing successively higher amounts of the emissivity material, are applied onto a nickel-aluminum interlayer. The balance of material in each coating layer is nickel-aluminum. Although the specimen shows improved thermal shock resistance, the composition of the layers are still discontinuous at the interlayer/coating and coating/coating interfaces. This limits their utility with materials of greatly differing thermal expansion values.
It is therefore advantageous to overcome the limitations of the prior art and to provide a method for forming thermally and physically stable interfaces between materials with different physical properties.